1. Field of the Invention
The invention relates to processes for dispersing solid additives in mixtures with synthetic, thermoplastic, polymer and to products prepared therewith. One embodiment concerns dispersing solid additives into particulated polycarbonates or blending the additives and particulated polycarbonates with other thermoplastic resins.
2. Brief Description of the Related Art
The patent literature is replete with descriptions of methods and processes for preparing homogeneous mixtures of synthetic polymeric resins with a wide variety of solid phase additives. Representative of such descriptions is that set forth in the U.S. Pat. No. 4,649,168 (Kress et al.) issued on Mar. 10, 1987. Kress et al. disperses particles of polytetrafluoroethylene (hereinafter referred to for convenience as "PTFE") in aromatic polycarbonate resin based molding compositions. The dispersion is carried out, in brief, by admixture of aqueous emulsions of the two components followed by coagulation of the emulsion mixture. Coagulation may be carried out by spray-drying, freeze-drying or the addition of inorganic or organic salts, acids, bases or organic solvents which are miscible with water.
The process described by Kress et al. does result in fine dispersions of the PTFE in the polycarbonate resin, but the degree of dispersion is dependent upon a number of variables, which require close control. For example, when two aqueous emulsions are mixed together, the way that these two emulsions will mix will be predominantly determined by the interactions of each of the suspended particulates with the water phase that they are emulsified with. This means that the surfactants necessarily used in each phase become dominant factors. During the process of coagulation, control of the dispersion of these two emulsions will become increasingly controlled by the interactions of the surfaces of the PTFE particle with the polymer resin particle. Since it is well known that like tends to repel like, there is an inherent driving force to cause segregation of these two mixed emulsions during coagulation.
Furthermore, when water is added to a resin powder the water becomes physically bound to and wets the surface of the powder. The amount of water that will be physically bound to this surface will be determined by the free energy characteristics of the powder surfaces, the amount of surface area available, and the free energy of interaction between water and this surface. Adding a water based emulsion of PTFE to an insufficiently wetted powder will result in the water immediately being stripped from the added emulsion, as long as the amount of water added is less than the saturation capability of the powder. This will result in an effective coagulation of the PTFE emulsion, because the emulsion cannot exist when the water is stripped from it. Since each particle of resin powder has a defined saturation potential for water, this will result in each resin powder particle only reacting with the water in a very small, defined amount of emulsion. This means that there is a forcing of coagulation of the PTFE emulsion on a distance scale determined by the water wetting characteristics of the resin powder. It also means that this will provide an extraordinary lack of dispersion of the PTFE because as soon as the surface of a particle of resin powder has been saturated or wetted out by water it no longer will have any capability to cause the coagulation of any more PTFE emulsion. This then becomes a physical limiting factor that will guarantee that the PTFE will be dispersed in the powder at a distance scale corresponding to the size of the PTFE emulsion size.
The process of the present invention is particularly advantageous in that the variables for controlling dispersion are limited. The process of our invention is simple, resulting in consistently good high dispersions on a scale not previously experienced. The highly homogeneous dispersions obtained produce products of a unique character and with superior properties. High proportions of additive are uniformly dispersed, without sacrificing mechanical properties. For example in regard to dispersing PTFE in polycarbonate resins, higher concentrations of the PTFE can be dispersed without difficulty and without compromising mechanical properties.
The process of the present invention incorporates PTFE into the particle structure of porous polycarbonate resins. Early processes for incorporating PTFE into polycarbonate resins included admixture and processing of the polycarbonate and the PTFE (in a latex) in a ball mill, a rubber mill, an extruder or a Banbury mixer; see for example the description in U.S. Pat. Nos. 3,294,871 and 3,290,795. In U.S. Pat. No. 3,005,795, it is suggested that admixture of the PTFE and the polycarbonate resin take place in "highly-fluid solutions and melts" of the base resin.
More recently, it has been found advantageous to combine the PTFE and a polycarbonate resin by co-precipitation of the two resins. This was said to result in the ultimate in dispersion of the PTFE into the polycarbonate resin; see the description in the U.S. Pat. No. 4,753,994 (Carter, Jr. et al., issued Jun. 28, 1988).
In this teaching, an aqueous dispersion of PTFE is admixed with a solution of polycarbonate resin and caused to co-precipitate by the addition of an ester, aromatic hydrocarbon, aliphatic hydrocarbon, alcohol, carbonate, ether or ketone. The coprecipitate then needs to be isolated, washed and dried in a somewhat costly recovery.
However, these prior art processes do not yield products with the unique properties, especially in maintaining mechanical properties, associated with the products of our inventive process which is inherently simpler, less time consuming and less costly.